Hi Dave

Good post.

you need to have channel A on B1S1 and channel B on Injector Pulse.

You have confirmed the HEGO sensors are working fine so, when the volts drop down to zero they are indicating the engine must be running very lean. when the car returned and you confirmed the DTC P0420 was this the only DTC Stored.

Because it as gone into open loop there will be not correction.

check injector PW when in closed open and see what happens to it when in Open loop.

when at idle and in closed loop what is the C0% reading then. I would expect around 0.50or a little lower.
Also have you a simulator if yes set it to DC volts and put 0.7 volts on the signal wire on B1S2. But get the engine upto temp first.

this May fool the ECU into thinking the CAT is OK , and good enough for static testing . i.e no vehicle speed or drive cycles completed.

keep the info coming and the answer will come